1. Field of the Invention
This invention relates to vacuum sheet feeding apparatus, and more particularly to a valve for controlling the vacuum supply for an oscillating vacuum sheet feeder.
2. Description of the Prior Art
In high speed printing apparatus or electrophotographic copier/duplicators, it is common pratice to feed precut sheets from a stack seriatim. The sheet feeding precut sheets from a stack seriatim. The sheet feeding apparatus may be either of the mechanical or the vacuum type. Mechanical feeding apparatus, such as scuff feeders or belt transports, require frictional interaction with the sheet to induce sheet movement. Vacuum feeding apparatus on the other hand, use pneumatic forces to maintain a sheet in contact with the feeder as the feeder is moved. One of the advantages of using vacuum feeding apparatus is that such apparatus may be more easily controlled to better assure pickup of a single sheet from a stack, decreasing the possibility of plural sheet feeds.
One type of vacuum feeding apparatus which is extremely efficient for feeding sheets at a desired high rate of speed is an oscillating vacuum feeder. The oscillating vacuum feeder generally includes an oscillating cylindrical housing which is connected to a vacuum source. The housing has a series of ports which permits the vacuum to be effective to tack one sheet from a stack of sheets to the housing. The oscillation of the housing feeds the tacked sheet from the stack into engagement with a nip roller in contact with the housing at a point remote from the stack. Circumferential bearings supported by the housing permit the sheet to continue in its feed path during return oscillation cycle of the housing (to return the vacuum ports to the point where feed of the next sheet from the stack is initiated). An example of such an oscillating vacuum feeder is shown in U.S. Pat. No. 2,770,458.
Under certain circumstances the oscillating vacuum feeders of the type described above have been ineffective for reliable high speed sheet feeding. Particularly the pneumatic forces exerted on the sheets being fed by the vacuum during the return oscillation cycle may disrupt the sheet feeding by creating undue drag forces on the sheets or by inducing excess static charge generation on the sheets. In order to overcome these potential problems, some prior art apparatus have employed mechanisms for interrupting the vacuum during the return oscillation cycle (see for example U.S. Pat. No. 3,764,255). Such apparatus has heretofore been of complex configuration requiring complicated electromechanical cut-off valves or intricate internal valve construction within the oscillating vacuum feeder housing.